The invention relates generally to the packaging of optical sensors, and more particularly to packaging optical sensors utilizing flip chip packaging techniques.
Devices such as digital cameras, displays, and scanners typically utilize an integrated circuit that generates electrical signals in response to incident light. The integrated circuit may include optical sensors, such as CCDs or active pixel sensors. In order for the integrated circuit to transport the generated electrical signals to supporting systems, the light sensitive integrated circuit is packaged and interconnected to a circuit board.
FIG. 1 is a depiction of a known optical sensor package arrangement. The optical sensor 104 is formed on a first substrate 106. The optical sensor receives light 108 and generates electrical signals in proportion to the intensity of the received light and then transmits the electrical signals to conductors within the first substrate. The first substrate is typically an integrated circuit that has electrical interconnections between each pixel in the optical sensor and contact pads 110 that are formed on the surface of the first substrate. The contact pads on the surface of the first substrate allow electrical connections to be made to a larger scale package.
The first substrate 106 is physically attached to a second substrate 112 such as an injection molded plastic, a glass reinforced laminate, or a ceramic package. The contact pads on the first substrate are electrically connected to bonding pads on the second substrate by very fine bonding wires 114. The optical sensor 104 and the first substrate 106 are typically covered with an optical glass cover 116 in order to seal the optical sensor within a package and protect the optical sensor from the outside environment.
The sealed package is then electrically connected to contact pads of a circuit board 118 by package leads 120 and solder bonds 122. The circuit board connects the optical sensor to other systems, such as memory and processors that are needed to support the desired functionality. In applications such as hand held digital cameras or scanners, a lens 130 may also be located above the glass cover and the optical sensor in order to focus incoming light 108 onto the optical sensor.
Although prior art optical sensor packaging techniques work well, as the size of electronic devices, especially hand held devices, shrinks, there is a need to package the optical sensors in smaller packages. The need for smaller packaging is especially important in the height dimension of the package, as measured from the lens to the circuit board. That is, it is desirable to provide optical sensors inxe2x80x9cthinnerxe2x80x9d packages.
One technique that has been utilized to package non-light sensitive integrated circuits with a large number of electrical connections is a ball grid array (BGA) mounting technique. Utilizing the BGA mounting technique, an integrated circuit is connected to a BGA package and the BGA package is physically and electrically connected to a circuit board with solder balls. The solder balls are soldered to contact pads on the BGA package and to corresponding contact pads on the circuit board. When a large number of electrical contacts is required between the BGA package and the circuit board, the solder balls are placed in a closely spaced array. As is known in the prior art, a solder reflow process is typically utilized to connect the solder balls to the conductive contact pads of the BGA package and to the conductive contact pads of the circuit board. Although the BGA mounting technique works well, the BGA mounting technique is generally utilized for packaging and mounting non-light sensitive integrated circuits, such as microprocessors and application specific integrated circuits.
In view of the desire for smaller electronic devices, such as cameras, displays, and scanners, what is needed is a packaging technique that allows optical sensors to be packaged in thinner packages.
An optical device packaging technique involves an optical sensor that is formed on a first substrate and flip chip bonded to a second substrate. The second substrate includes a through hole or a transparent material that is aligned with the optical sensor in order to allow light to contact the optical sensor. By flip chip bonding the first substrate to the second substrate, the optical sensor is provided in a thin package that is useful in many applications, such as hand held cameras.
A first embodiment of an optical device structure includes an optical sensor, a first substrate, a second substrate, and a circuit board. The optical sensor is formed on or within the first substrate and the individual sensors or pixels of the optical sensor are electrically connected to contact pads that are exposed on the top surface of the first substrate. The first substrate is connected to the second substrate by solder balls that connect contact pads on the bottom surface of the second substrate to corresponding contact pads on the first substrate. The second substrate is a package substrate, such as an injection molded plastic substrate, a ceramic substrate, or a glass reinforced laminate. The second substrate includes electrical conductors that electrically connect the first set of contact pads to a second set of contact pads on the periphery of the second substrate. In an embodiment, the second substrate includes a through hole that allows light to contact the optical sensor.
The second substrate is connected to the circuit board with interconnects, such as solder balls, as is known in the art of ball grid array mounting. The interconnects provide an electrical connection between a second set of contact pads on the second substrate and corresponding contact pads on the circuit board. As a result of the various levels of electrical interconnections, the individual light sensitive devices of the optical sensor are electrically connected to the conductive contact pads of the circuit board in a thin package.
The first embodiment may include an optical glass cover that is placed over the opening in the second substrate in order to protect the optical sensor from damage while allowing light to contact the optical sensor.
The first embodiment may include an optical grade structural adhesive that is applied directly over the optical sensor instead of an optical glass cover that is attached to the second substrate.
The first embodiment may include an optical lens system that is placed over the opening in the second substrate in order to enable focusing of light onto the optical sensor.
The first embodiment may include an optical glass substrate as the second substrate. The optical glass substrate may include metal patterning on the bottom surface that provides a portion of the electrical connection between the optical sensor and the circuit board. The first embodiment may include an optically transparent glass structure that includes an integrated lens element, or multiple elements, as the second substrate.
The first embodiment may be incorporated into a camera housing that includes an opening for allowing light to contact the optical sensor.
A second embodiment of an optical device structure includes an optical sensor, a first substrate, and a circuit board as the second substrate. In the embodiment, contact pads of the first substrate are connected to contact pads of the circuit board with conductive balls. The connection between the first substrate and the circuit board provides the electrical connection between the optical sensor and the circuit board. In an embodiment, the circuit board includes a through hole that allows light to contact the optical sensor. A transparent circuit board may alternatively allow light to contact the optical sensor.
The second embodiment may include an optical glass cover that is placed over the opening in the circuit board in order to protect the optical sensor from damage while allowing light to contact the optical sensor.
The second embodiment may include an optical grade structural adhesive that is applied directly over the optical sensor instead of an optical glass cover that is attached to the circuit board.
The second embodiment may include an optical lens system that is placed over the opening in the circuit board in order to enable focusing of light onto the optical sensor.
The second embodiment may include a monolithic glass structure that includes a lens element or multiple lens elements as the circuit board. In an embodiment, the monolithic glass structure forms part of a camera housing.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principals of the invention.